In information-processing apparatuses, such as personal computers, word processors, etc., magnetic disk systems are widely used as external data memory units. One such magnetic disk system is disclosed in Japanese Patent Disclosure No. 57-36473. This prior art system is a so-called micro-floppy disk drive system, in which micro-floppy disks with a diameter of less than 5.25, usually 3.5 inches, are used as data-recording media.
Unlike mini-floppy disks of 5.25-inch diameter or standard floppy disks of 8-inch diameter, micro-floppy disks are generally enveloped in a hard casing of synthetic resin, and also have a metallic center hub in their central portion. The center hub has a first hole in its central portion, and a second hole in a position offset from the first hole. The second hole is used in the driving and centering of the micro-floppy disk. When the disk is set on the drive unit, a spindle of the unit is inserted into the first hole, and an urging pin, which is adapted to be rotated in synchronism with the spindle, is fitted into the second hole. With this construction, the disk is rotated around the spindle, while being urged radially outward. In this way, the disk is centered, and is rotated in synchronism with the rotation of the spindle or the urging pin.
In the prior art micro-floppy disks, as described above, the center hub must be formed with first and second independent holes. As for the drive unit for the disk, it requires the urging pin, which is rotated in synchronism with the spindle. This makes the unit complicated in construction.
In the conventional drive unit, moreover, the urging pin is attached to a leaf spring, in order to facilitate the insertion of the pin into the second hole, and to securely urge the micro-floppy disk in the radial direction, by means of the pin. For smoother sliding contact between the urging pin and the inner edge of the second hole, furthermore, a rotary bearing is attached to the end tip of the pin. Thus, the prior art drive unit, in which the disk is rotated and centered by the joint action of the urging pin and second hole, additionally requires not only the pin, but also the spring and bearing. Consequently, the construction of the unit is further complicated.